Dear Alex,
Dear Alstan,
> Are you sure your low dynamic range pictures are properly exposed from
> which you construct the HDRI?
>
> Meaning, the fastest shutter speed picture has no saturation (i.e. all
> white pixels) and your slowest shutter speed picture has some black
> pixels? You need to make sure this is true else you won't get proper
> illuminance calculation and also your bright spots (i.e. high
> luminance) will be under reported.
Well, I took 15 images with auto-bracketing (1EV between each image). I always
check that the fastest shutter speed is almost all black pixels (no white
pixels) and the slowest shutter speed is almost all white pixels (no black
pixels). If I have too high luminance in my field of view, then I use a neutral
density filter so that my darkest image does not have any white pixel.
> I'd like to follow up with some other potential things to look out for in
> addition to what Alex suggested and questions based on your description. We
> dealt a lot
> with this via the papers on HDR we published last year.
Yes, I have read it.
> * Full fisheye HDR images should always be cropped to a bounding
> square and the view edited or input inline via Evalglare when doing
> analysis. Are you cropping the images in all cases?
Yes, all images are cropped at the end (after hdrgen and vignetting
calibration) to a 1000x1000 square (with pcompos and pfilt) and the VIEW line
in the header is modified to a -vta -vh 180 -vv 180.
> * Where is your 1.39 calibration factor derived from? It is useful to
> measure luminance for every HDR photo taken from an easy to identify
> (and nearly-neutral) surface for use in calibrating images rather
> than a constant. Typically the discrepancies I find when I
> /don't///measure luminance this aren't as high as what you found
> however.
I derived my calibration factor when doing the vignetting calibration process.
I did the vignetting calibration according to the method suggested in Cauwerts
et al. (2013) paper. Therefore I have a calibration factor for each aperture:
f/3.5 --> 1.32
f/5.6 --> 1.38
f/11 --> 1.48
f/16 --> 1.59
f/22 --> 1.84
The reason why the f/3.5 calibration factor is not 1.39 anymore, is because I
noticed that using hdrgen command with -e and -a options when creating my HDR
images, gave me better results. It changes the exposure of the generated HDR
images and I thus only got differences between measured and HDR-derived values
of max 30% (it was 50% before). Therefore, I recalculated my calibration
factors with the last generated HDR images.
> * You should look up the recent discussion, "[HDRI] Convert
> equisolidangular to equiangular projection" which strongly suggests
> the Sigma 8mm f/3.5 to be a -vta / equidistant / equi-angle lens.
> Reprojecting the image could add some error in this case.
Well, I think this is not the case. The SIGMA 8mm f/3.5 has an equisolid
projection.
I am quite sure as I received this information from Sigma UK. Moreover, I
checked the information Sigma gave me by measuring myself the projection of the
lens. I did it following David Geisler-Moroder method, that he presented at the
15th International Radiance Workshop in 2016. And I also got an equisolid
projection. I could send you more info if you want.
The images have thus to be reprojected from equisolid to equidistant.
Best,
Clotilde
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Envoyé : jeudi 9 février 2017 23:47
À : [email protected]
Objet : HDRI Digest, Vol 88, Issue 7
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Today's Topics:
1. Re: Illuminance and luminance values underestimation from
calibrated HDR images (J. Alstan Jakubiec)
----------------------------------------------------------------------
Message: 1
Date: Fri, 10 Feb 2017 06:45:43 +0800
From: "J. Alstan Jakubiec" <[email protected]>
To: [email protected]
Subject: Re: [HDRI] Illuminance and luminance values underestimation
from calibrated HDR images
Message-ID: <[email protected]>
Content-Type: text/plain; charset="windows-1252"; Format="flowed"
Hello Clotilde,
I'd like to follow up with some other potential things to look out for in
addition to what Alex suggested and questions based on your description. We
dealt a lot with this via the papers on HDR we published last year.
* Full fisheye HDR images should always be cropped to a bounding
square and the view edited or input inline via Evalglare when doing
analysis. Are you cropping the images in all cases?
* Where is your 1.39 calibration factor derived from? It is useful to
measure luminance for every HDR photo taken from an easy to identify
(and nearly-neutral) surface for use in calibrating images rather
than a constant. Typically the discrepancies I find when I
/don't///measure luminance this aren't as high as what you found
however.
* You should look up the recent discussion, "[HDRI] Convert
equisolidangular to equiangular projection" which strongly suggests
the Sigma 8mm f/3.5 to be a -vta / equidistant / equi-angle lens.
Reprojecting the image could add some error in this case.
Best,
Alstan
On 2/9/2017 6:09 AM, Alex Mead wrote:
> Clotilde:
>
> Are you sure your low dynamic range pictures are properly exposed from
> which you construct the HDRI?
>
> Meaning, the fastest shutter speed picture has no saturation (i.e. all
> white pixels) and your slowest shutter speed picture has some black
> pixels? You need to make sure this is true else you won't get proper
> illuminance calculation and also your bright spots (i.e. high
> luminance) will be under reported.
>
> - Alex
>
> On Wed, Feb 8, 2017 at 11:10 AM, Clotilde Pierson
> <[email protected] <mailto:[email protected]>>
> wrote:
>
> Dear all,
>
> I am using a Canon EOS 5D Mark II with a Sigma EX DG 8mm f/3.5
> fisheye lens to capture LDR images. I then use hdrgen with the
> response curves I defined previously to generate HDR images from
> these LDR images. I calibrate the HDR images for the vignetting
> effect and the distortion (equisolid to equidistant projection).
> Finally, I apply a calibration factor of around 1.39.
>
> When comparing the vertical illuminance and luminance values of
> the HDR images with the real measures I took (Minolta LS-110
> luminancemeter and Hagner EC1-X luxmeter), I noticed that the HDR
> images are underestimating the illuminance (e.g. 1217lux instead
> of 1993lux) and the high luminance values (e.g. 208 cd/m? instead
> of 402.3cd/m?). I determined the illuminance value of an HDR with
> Evalglare ?V and the luminance values with ximage in Radiance.
>
> I also tried only applying default hdrgen (+cropping & header
> modification to set the VIEW to vta to use in Evalglare) but I
> still got big differences between the HDR-derived and the measured
> luminance and illuminance values.
>
> Is somebody using the same instruments I am? If yes, do you also
> happen to have this issue? Or does anybody have already encounter
> this problem?
>
> Thank you for your insights !
>
> Best,
>
> Clotilde
>
> *Test*
>
> **
>
> *Clotilde Pierson*
>
> /FNRS PhD Fellow | Arch. Eng./
>
> *Architecture et Climat**
> *Facult? d?architecture, d?ing?nierie architecturale et
> d?urbanisme (LOCI)
>
> Universit? catholique de Louvain (UCL)
> Place du Levant, 1 bte L5.05.04 B-1348 - Louvain-la-Neuve (Belgique)
> [email protected]
> <mailto:[email protected]>
>
> T?l. 32 (0)10 47 91 52 - Fax 32 (0)10 47 21 50
> http://www.uclouvain.be/architecture-climat.html
> <http://www.uclouvain.be/architecture-climat.html>
>
>
> _______________________________________________
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>
>
>
>
> --
> Alex Mead
> (616) 901-2479, UC Berkeley Systems Engineering Ph.D. (expected 2017),
> www.alex-mead.com <http://www.alex-mead.com>
>
> www.CEEphotos.com <http://www.ceephotos.com> - web master, creator
>
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